Relative roles of glyceollin, lignin and the hypersensitive response and the influence of ABA in compatible and incompatible interactions of soybeans with Phytophthora sojae

The relative roles of glyceollin, lignin and the hypersensitive response (HR) in pathogen containment and restriction were investigated in soybean (Glycine max L. [Merr.]) cultivars that were inoculated with Phytophthora sojae Kaufmann and Gerdemann. Concentrations of endogenous abscisic acid (ABA) levels in etiolated soybean hypocotyls were reduced by application of the ABA biosynthesis inhibitor norflurazon or raised by exogenous ABA application. Incompatible interactions in leaves and hypocotyls were characterized by HR, phenolic and lignin deposition and glyceollin accumulation. Compatible interactions resulted in light coloured, water-soaked spreading lesions with minimal lignin deposition or glyceollin accumulation and the absence of an HR. Norflurazon treatment restricted the spread of the pathogen and increased glyceollin accumulation in compatible tissues. Exogenous ABA addition caused spreading lesions in normally incompatible interactions and reduced glyceollin accumulation. Phenolic deposition and HR were unchanged by either treatment in incompatible or compatible interactions. The uncoupling of glyceollin synthesis from the HR and phenolic and lignin deposition by ABA and norflurazon treatment showed that glyceollin is a major factor in restriction of the pathogen during these interactions.